A chuck has been developed for holding and gripping easily distortable workpieces. Such a chuck is disclosed in the present assignee's U.S. Pat. No. 4,953,877 to Slachta et al, granted Sep. 4, 1990, which is incorporated herein by reference, and employs a plurality of discrete flexible chucking fingers actuated by a pliable, inflatable bladder ring for radially deflecting the fingers conjointly into engagement with the peripheral wall of the workpiece. With some workpieces, where the workpiece includes a rigid hub wall which is pre-centered with respect to the machine spindle, it has been found necessary to grip a non-concentric wall of the part eccentrically, and a chuck of the type disclosed in the patent has been improved to achieve the result desired.
In contrast, the present collet chuck is provided with generally radially movable positioning fingers for centering a non-concentric workpiece, prior to fully gripping it with concentric collet fingers. The chuck is useful, inter alia, for exteriorly gripping workpieces such as spherical bearings, which are formed of a pair of mating halves which have their abutting edges brazed and may be as much as thirty to sixty thousandths out of concentricity, and securely holding them for interior wall machining.